1. Field of the Invention
The present invention relates to an etching solution for etching copper or copper alloy in an electronic substrate, in particular, an etching solution for selectively etching copper or copper alloy in an electronic substrate having an electrode (bump) made of copper or copper alloy and nickel.
2. Description of the Related Art
In order to improve the performance of electronic devices, their elements have been miniaturized or mounted with high density. However, particularly in semiconductor devices, the miniaturization technology is approaching its limits. Devices with a three-dimensional structure have been put to practical use by using conventional wire bonding, flip chip or bump as a technology of mounting elements with high density. However, there is a demand for further increase in the density. Therefore, TSV technology has been being developed, which is a technology of forming a thin via penetrating through silicon and filling such a via with a conductor such as copper to form an electrode (Non-Patent Document 1).
When the copper is used as an electrode in the TSV technology, it is the common practice to make a hole in a silicon substrate, form a barrier metal layer such as silicon oxide film or titanium film on the inner wall of the hole, and then form a copper seed layer by the metal organic chemical vapor deposition or physical vapor deposition (FIG. 1). Then, a protecting film is formed using a resist resin on the copper seed layer except a portion on which an electrode is to be formed (FIG. 2). The portion where no protecting film is formed is filled with a metal such as copper to form a bump. However, since an exposure of copper deteriorates the connection reliability due to causing a surface oxidizing phenomenon, it is the common practice to stack a nickel layer and a solder layer made of an alloy of gold or tin and silver (FIG. 3). Then, the resist resin is removed to form the bump (FIG. 4).
The copper seed layer and the barrier metal layer are formed not only in the hole of the silicon substrate but also on the surface of the silicon substrate. Even after removal of the resist, they still remain. Therefore, they must be removed using an etching solution (FIG. 5).
As a method for wet etching of the copper seed layer, a method using an etching solution consisting of an acid and an oxidizing agent, for example, a mixture of sulfuric acid and hydrogen peroxide is widely used (Patent Document 1). Also, a etching method using an etching solution containing copper chloride or ferric chloride is widely known (Patent Document 2). Further, a etching method using an etching solution composed of sulfuric acid, hydrogen peroxide, and a polyethylene glycol derivative serving as a surfactant is also widely known.
However, in the etching methods as disclosed in Patent Documents 1 to 3, when the copper seed layer formed on the electronic substrate is etched after bump formation, there is a problem of causing a deform of the bump for the reason that a nickel used for the bump formation is etched.
In the existing etching devices, the liquid level of a buffer tank containing a chemical solution is detected and controlled by a sensor and thus generated foam from the etching solution causes the sensor to malfunction. In addition, the etching solution is circulated using a pump and thus the generated foam causes air entrainment of the pump, leading to a problem in the delivery of the etching solution. Therefore, in the etching method as disclosed in Patent Document 3, foam is generated, which disturbs smooth work.    [Non-patent Document 1] “TSV Technology for Three-dimensional Package” (written by Seiichi Denda, published by Kogyo Chosakai Publishing Co., Ltd, 2009), pp. 12 to 16.    [Patent Document 1] Japanese Patent Laid-Open No. 2000-286531    [Patent Document 2] Japanese Patent Laid-Open No. 2008-285720    [Patent Document 3] Japanese Patent Laid-Open No. 2009-120870